Known moving bed ion-exchange apparatus can be either continuous or discontinuous, with a fluidized or a compact bed.
In one apparatus, of the parallel-columns type, the columns are connected at their ends through curvilinear sections, ion-exchange resin in the loading zone is gradually saturated in small increments after which the saturated resin portion is moved upward by hydraulic ram action into a backwash zone; from the latter it is passed into a pulse zone and is further transferred into a regeneration zone thereby entering a rinse zone from which it is returned to the saturation zone.
With this ion-exchange apparatus, the regeneration and rinse zones are not separated by valves and are dimensioned so as to receive additional portions of saturated resin.
Water introduced into the rinse zone dilutes the regenerant at the zone contact interface. Likewise, the pulse water dilutes the regeneration effluent at the contact interface.
In order to avoid the dilution of regenerant and the regenerating effluent and to keep the interface at a steady state (rinse water -- regenerant -- pulse water -- regeneration product) it is absolutely necessary to control each pulse cycle, with automatic valves operated by interface analyzers. Because it is necessary to maintain constant level control of separating interfaces, rinse water and regenerating agent flow rates must be constant, which requires the removal of saturated resin portions from the loading zone at a constant frequency controlled in accordance with a predetermined time schedule.
Under the circumstances required by the treatment of salt-rich and time-variable composition liquids and by the use of highly concentrated regenerants, and with respect to the need for minimizing the degradation of physico-chemical characteristics of the ion-exchange resins, this apparatus has a number of disadvantages, such as:
the need for buffer tanks with large volumes for homogenizing and maintaining the influent composition constant;
degradation of cationic resins due to the high temperature at the contact interface between rinse zone and regeneration zone on account of the high exothermicity of the dilution of strong acids with wash water;
dilution of regeneration effluent and lowering of loading rate in case of interface analyser malfunction within the regeneration-rinse zones; and
significant mechanical degradation of ionic resins in consequence of high operating pressure in the regeneration-rinse zones as well as of resin swelling in the case of using carboxyl-cation or weak base and strong base anion-exchange resins in the saturation zone, where high internal pressures result where the spaced is limited by valves.